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Thermodynamics Notes for Class 11 - 12

Principle of Measurement of Temperature

Thermometric Property

The property of any matter that can be used for measuring temperature is called thermometric property. Examples of thermometric properties are altitude of liquid in a capillary tube, pressure of gas in constant volume or a certain volume of gas in constant pressure, resistance of conductors or semiconductors, etc.

Thermometric Substance

The substances whose thermometric properties are used for making thermometers are called thermometric substances.

Thermal Equilibrium

When two objects of different temperatures come in contact with each other, they reach the same temperature and this situation is called thermal equilibrium.

Zeroth Law of Thermodynamics

If two objects are in thermal equilibrium with third object separately, the first two objects will be in thermal equilibrium with each other.


Temperature is the thermal condition of an object that determine wheather the object will absorb heat or release heat if it comes in contact of another object.


The instrument with which the temperature of an object and difference in temperature among the different objects can be determined is called thermometer.

Lower fixed point

The temperature at which dry ice reaches thermal equilibrium with water, or the temperature at which pure ice begins to melt is called lower fixed point.

Upper fixed point

The temperature at which pure water reaches thermal equilibrium with steam at standard pressure, or the temperature at which pure water converts into steam is called upper fixed point.

Triple point of water

At 4.58 mm Hg pressure pure ice, water and steam reach thermal equilibrium. The temperature at which these three phases can coexist is called triple point of water.

First Law of Thermodynamics

First Law of Thermodynamics

James, Joules, a scientist, performed a lot of experiment relating heat and mechanical energy. He firstly presented his findings in the form of a formula in 1849. This is known as the first law of thermodynamics.

Significance of the first law of thermodynamics

1) It establishes the relationship between heat and work.

2) Work and heat are equivalent to each other and conservation of energy is maintained during the convertion work into heat or vice-versa.

3) Heat applied to system  equals the sum of work done and change in internal energy.

4) It is impossible to have work done without using energy.

5) According to this law, a fixed amount of heat is required to obtain a fixed amount of work or vice-versa.

Isobaric Process

The process in which pressure in a system remains constant is called isobaric process.

Isothermal Process

The process in which the temperature of a gas always remains constant is called isothermal process. If the volume of a gas is expanded or contracted at constant temperature, the change of volume is called isothermal expansion or isothermal contraction and the process is called isothermal process.

Adiabatic Process

The process in which no heat is received by the system from outside or no heat is released from the system is called adiabatic process.

Characteristics of Adiabatic Process

1) No heat is exchanged in this process. Therefore, dQ = 0.

2) It is fast process.

3) Adiabatic curve is steeper than isothermal curve.

4) The specific heat of gas in adiabatic process is zero.

Isochronic Process

The process which changes heat energy or internal energy of gas but does not change the volume of the system is called isochronic process.

Thermal System or Thermodynamic System


The small definite number of matters which we consider for conducting an experiment is called a system.

Every system belong to a definite volume, a definite mass and a definite internal energy. Systems can be of different types such as open system, closed system and isolated system.

Open System: The system which can exchange both mass and energy with its surroundings is called an open system.

Closed System: The system which can exchange only energy but can not exchange mass with its surroundings is called a closed system.

Isolated system: The system which is not affected by its surroundings, that is, which does not exchange either mass or energy with its surroundings is called an isolated system.

Thermal System

Thermal system is a component of the material world enclosed by a certain surface or boundary. For example, gas enclosed in cylinder with a piston or in a balloon is a thermal system.


The matters around a system is called its surroundings.

Thermodynamic Process

The change for which the thermodynamic co-ordinates of a system is changed is called thermodynamic process.

Thermal equilibrium

Steady state of an isolated system is called thermal equilibrium.

Internal Energy

Internal energy

Internal energy of every object can do work and can be converted to other energies. Energy produced due to linear motion, vibratory motion and rotational motion of molecules, atoms and fundamental particles and their forces is called internal energy.

Dependence of internal energy

The internal structure of a system is an important factor of internal energy. If the internal structure of a system remains unchanged, internal energy depends on volume (V), pressure (P) and temperature (T) along with some other physical properties like specific heat, co-efficient of expansion, etc.

Dependence of internal energy of a system in terms of a few cases is mentioned below:

1) For an ideal gas magnitude of thermal kinetic energy is the internal energy. This value depends on the temperature of the gas and the number of molecules. It does not depend on pressure or volume. This phenomenon is known as Mayer's hypothesis.

2)  For liquid substances, kinetic energy of the molecules refers to its internal energy.

3) For solid substances, kinetic energy due to vibration of molecules at rest position is the internal energy.

Internal energy changes of any gas depends on only its temperature but it does not depend on its volume or pressure.

Internal energy of an object can be increased by doing work.

Heat,Internal Energy and Work

Let, the application of ΔQ amount of heat to a system increases internal energy by ΔU and at the same time ΔW amount fo work is done by the system. So according to the first law of thermodynamics,

ΔQ = ΔU + ΔW

This is the general representation of the first law of thermodynamics because of the reason that infinitesimal change, dQ = dU + dW.

The sign of ΔQ, ΔU and W are given below:

 Positive(+) Negative(-)
ΔQ When heat is applied to system.
When system loses heat.
ΔU When internal energy of system increases.When internal energy decreases.
ΔW Work done by system.Work done on system.

What is specific heat?

The amount of heat is needed to increase temperature of 1 kg of substance by 1 K is called specific heat.

What is molar specific heat or molar heat capacity?

To define specific heat for gas, 1 mole is used instead of 1 g or 1 kg. The related specific heat is called molar specific heat or molar heat capacity.

To increase ΔT temperature of m mole of gas if ΔQ amount of heat is needed, molar specific heat, C = ΔQ ÷ mΔT

Unit: Unit of molar specific heat is Jmol-1K-1

Types of specific heat: A gas has two form of specific heat:

a) Molar specific heat at constant pressure, Cp

b) Molar specific heat at constant volume, Cv.

a) Molar specific heat at constant pressure, Cp: At constant pressure, the amount of heat needed to increase temperature of 1 mole of gas by 1 K is called molar specific heat at constant pressure. So, molar specific heat at constant pressure, Cp = ΔQ ÷ mΔT

b) Molar specific heat at constant volume, Cv: At constant volume the amount of heat needed to increase temperature of 1 mole of gas by 1 K is called molar specific heat at constant volume. So, molar specific heat at constant volume, Cv = ΔQ ÷ mΔT

Value of Cp is greater than that of Cv - Explain.

If heat is applied to a gas keeping its volume constant, temperature and pressure of the gas increase. For this reason heat is needed 

Reversible process

The process which comes back to its initial position and in each step of forward and backward processes the amount of heat and work done are equal and opposite to each other is called reversible process.

Irreversible process

The process which cannot come back to its initial position and if in every step of forward and backward processes the amount of work is different is called irreversible process.

Carnot's engine

To convert heat energy into mechanical energy Carnot proposed an ideal engine which is known as Carnot's engine.

Carnot's cycle

The cycle in which an ideal heat engine can continuously convert energy is called Carnot's cycle.

Principle of producing heat engine

It is impossible to produce an engine that can entirely convert the thermal energy absorbed from heat source into work in reversible process.

Efficiency of engine

The ratio of amount of thermal energy converted into work by an engine in terms of the amount of thermal energy absorbed is called efficiency of engine.


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